Bidirectional printing
Laying down characters on the head's return sweep too, instead of treating the trip back as dead travel.
A dot matrix printer that only prints left to right spends half its life driving an empty carriage the other way. Printing in both directions nearly doubles throughput and costs nothing in hardware — just enough buffer to hold the whole line and the logic to clock it out backwards. Epson shipped it in the MX-80 in 1980, and it became standard on anything with a head on a carriage, inkjets included.
The price is registration. The carriage has slop, the belt stretches, and the head fires with a small delay, so the two directions land a few hundredths of a millimetre apart. On body text you barely see it; on graphics and vertical rules it shows up as visible zigzag. That is why every such printer ships a bidirectional alignment routine, and why it drops back to unidirectional when quality matters more than speed.